The basic goal of this project is a better understanding of the structure of the Factor VIII molecule and its role in the regulation of hemostasis. Studies will be concentrated on 4 major features of the Factor VIII molecule: a) its basic structure, with especial reference to the carbohydrate portion, b) its procoagulant (antihemophilic factor) activity, c) its interaction with specific antibodies and lectins, and d) its platelet agglutinating properties. Four different approaches will be used to study these features: a) Factor VIII will be treated with proteolytic enzymes, and the resulting fragments purified and analyzed to determine their composition, activities, and relation to the intact Factor VIII structure. b) Factor VIII will be treated with specific glycosidases to sequentially remove carbohydrate residues. The amount of carbohydrate removed will be correlated with the remaining physiological and immunological activities. c) Factor VIII will be chemically modified with specific reagents and the effects on its activities determined. d) The nature of the platelet receptor for Factor VIII will be examined, along with a study of the metabolic response of the platelet to the interaction with Factor VIII. These studies are an attempt to gain a basic understanding of the structure of Factor VIII and the molecular mechanisms which are responsible for its regulation of blood clotting and platelet-mediated hemostasis. These studies will hopefully aid in the treatment of hemophilia and in the control of certain thrombotic disorders.